Docs: Link to the "Conditional Tags" article in the Theme Developer Handbook from the descriptions for a variety of core conditional tag functions.

These notations largely serve to direct consumers (of both the source and the parsed code reference) to extended information on individual and related conditional tags throughout WordPress. The changeset also standardizes corresponding DocBlock summaries to use third-person singular verbs.

Notations been added for the following functions:

* comments_open()
* email exists()
* has_excerpt()
* has_post_thumbnail()
* has_tag()
* in_category()
* in_the_loop()
* is_404()
* is_active_sidebar()
* is_active_widget()
* is_admin()
* is_admin_bar_showing()
* is_archive()
* is_attachment()
* is_author()
* is_blog_installed()
* is_category()
* is_comments_popup()
* is_date()
* is_day()
* is_dynamic_sidebar()
* is_feed()
* is_front_page()
* is_home()
* is_local_attachment()
* is_main_query
* is_month()
* is_multi_author
* is_new_day()
* is_page()
* is_page_template()
* is_paged()
* is_plugin_active()
* is_plugin_active_for_network()
* is_plugin_inactive()
* is_plugin_page()
* is_post_type_archive()
* is_preview()
* is_rtl()
* is_search()
* is_single()
* is_singular()
* is_sticky()
* is_tag()
* is_tax()
* is_taxonomy_hierarchical()
* is_time()
* is_trackback()
* is_user_logged_in()
* is_year()
* pings_open()
* post_type_exists()
* taxonomy_exists()
* term_exists()
* username exists()
* wp_attachment_is_image()
* wp_script_is()

Props janalwin.
Fixes #43254.

Built from https://develop.svn.wordpress.org/trunk@42710


git-svn-id: http://core.svn.wordpress.org/trunk@42538 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/pluggable.php

@@ -981,8 +981,12 @@ endif;
 
 if ( ! function_exists( 'is_user_logged_in' ) ) :
 	/**
-	 * Checks if the current visitor is a logged in user.
+	 * Determines whether the current visitor is a logged in user.
 	 *
+	 * For more information on this and similar theme functions, check out
+	 * the {@link https://developer.wordpress.org/themes/basics/conditional-tags/ 
+	 * Conditional Tags} article in the Theme Developer Handbook.
+	 * 
 	 * @since 2.0.0
 	 *
 	 * @return bool True if user is logged in, false if not logged in.